Perhaps the 3rd Brake light fuse is on the same fuse link off the Rear Defog. In the thread on the Sequential Tail Lights there is a post pointing to instructions on installing a 3rd Brake Light Pulser from Webelectric -
http://www.camaro5.com/forums/showpo...6&postcount=27
follow the link to the poster's instructions and you can see pics of the wiring harness - to me this looks like both the Rear Defog and 3rd Brake Light wiring is in the same harness.
Net of all this - check your rear defog to see if its working - if not then its on the same fuse which may be the issue.
Just a thought...